1. Field of the Invention:
The present invention relates generally to a simplified, reduced instruction set computer (RISC) microprocessor. More particularly, it relates to such a microprocessor which is capable of performance levels of, for example, 20 million instructions per second (MIPS) at a price of, for example, 20 dollars.
2. Description of the Prior Art:
Since the invention of the microprocessor, improvements in its design have taken two different approaches. In the first approach, a brute force gain in performance has been achieved through the provision of greater numbers of faster transistors in the microprocessor integrated circuit and an instruction set of increased complexity. This approach is exemplified by the Motorola 68000 and Intel 80X86 microprocessor families. The trend in this approach is to larger die sizes and packages, with hundreds of pinouts.
More recently, it has been perceived that performance gains can be achieved through comparative simplicity, both in the microprocessor integrated circuit itself and in its instruction set. This second approach provides RISC microprocessors, and is exemplified by the Sun SPARC and the Intel 8960 microprocessors. However, even with this approach as conventionally practiced, the packages for the microprocessor are large, in order to accommodate the large number of pinouts that continue to be employed. A need therefore remains for further simplification of high performance microprocessors.
With conventional high performance microprocessors, fast static memories are required for direct connection to the microprocessors in order to allow memory accesses that are fast enough to keep up with the microprocessors. Slower dynamic random access memories (DRAMs) are used with such microprocessors only in a hierarchical memory arrangement, with the static memories acting as a buffer between the microprocessors and the DRAMs. The necessity to use static memories increases cost of the resulting systems.
Conventional microprocessors provide direct memory accesses (DMA) for system peripheral units through DMA controllers, which may be located on the microprocessor integrated circuit, or provided separately. Such DMA controllers can provide routine handling of DMA requests and responses, but some processing by the main central processing unit (CPU) of the microprocessor is required.